1. Field of the Invention
The present invention generally relates to a method for wavelength stabilization of a liquid crystal display (LCD). More particularly, the present invention relates to a system and method for stabilizing wavelength of LED (light emitting diode) radiation in backlight module of the LCD.
2. Description of Related Art
An LCD includes a controllable transmissive display panel that faces users, and a backlight module that provides the controllable transmissive display panel with illumination from its rear side. The backlight module may employ LED or cold cathode fluorescent lamp (CCFL) as a light source. The LED backlight module has at least two advantages over CCFL backlight module; one is full color reproduction and the other is no contamination of mercury (Hg). During the period of manufacturing the CCFL backlight module, operators may be endangered if mercury contained in the CCFL is released. As such, the LED backlight module not only provides users with better color quality but also prevents the operators from being poisoned by mercury. Hence, the LED backlight module is promising to be a main stream of next generation of displays.
In the LED backlight module, a plurality of LEDs are arranged in a matrix form that illumines pixels of the controllable transmissive display panel. Since any color light is a combination of three prime colors; i.e. red (R), green (G) and blue (B) colors, every red LED, green LED and blue LED are grouped in order to illumine each pixel. For example, with a certain combination of R, G and B colors, there produces “white” light. However, the LED backlight module has some drawbacks. That is, aging of the LED backlight module and variation of environment temperature respectively incur light intensity attenuation and wavelength drift, degree of which are varied for the different LEDs with the same color. As shown in FIG. 1, as environment temperature changes from 34° C. to 78° C., wavelength of LED radiation shifts from shorter wavelength to longer wavelength. Thus, a circuit, capable of detecting light intensity and wavelength of each LED radiation and then proceeding to compensate them if they deviate from default values, is a crucial component for improving performance of the LED backlight module. However, currently, all color feedback systems for the LED backlight module compensate each produced color or light intensity of each LED radiation, rather than wavelength of each LED radiation. Since human eyes have different sensitivities for different wavelengths, even the same color light with different wavelengths causes human eyes to have different stimulus. Furthermore, conventional color sensors are only responsive to light intensity, rather than to offset of wavelength of each LED radiation. In other words, the conventional color sensors are not able to compensate variation of wavelength of each LED radiation even color feedback systems are employed, which causes the chromaticity coordinate of the LED backlight module to be drifted.
Additionally, as there exists parameter discrepancy in growth of epitaxy layer when manufacturing the LED, there are wavelength discrepancies among a batch LEDs with the same color. To avoid higher cost for batching LEDs with a wavelength range (hereinafter referred to as bin), nowadays the bin employs 5 nm as a minima bin range. However, the 5 nm bin incurs color shift perceived by human eyes. Thus, to overcome this color shift, a smaller bin is necessitated, which in turn increases the cost for batching LEDs. Moreover, as mentioned above, stability of the chromaticity coordinate of the LED backlight module is affected by the environment temperature.
There are some approaches to overcome aforementioned problems. For example, U.S. Pat. No. 7,220,959 discloses a differential color sensor 200 without filters. As shown in FIG. 2, two photodiodes 100, 150 are fabricated such that they have different sensitivities vs. wavelengths, wherein one has its sensitivity peak in shorter wavelengths, while the other has its sensitivity peak in longer wavelengths. The two photodiodes convert received light into voltage signals via resistors 120,170, and a voltage ratio between these two photodiodes is obtained via a divider 210. Based on the voltage ration, spectra content of incident light can be obtained. However, U.S. Pat. No. 7,220,959 is not able to calculate wavelength variation of radiation of these two photodiodes, and independently compensate wavelength variation for each one of these two photodiodes.
U.S. Pat. No. 6,678,293 discloses a wavelength sensitive device for wavelength stabilization. This wavelength sensitive device (i.e. photodiode) comprises a plurality of layers jointly defining two opposite diodes generating opposite photocurrents. Amount of the opposite photocurrents is determined in accordance with fabricating parameters of the two opposite diodes. That is, by using a certain doping ratio for the two opposite diodes, an output current of the photodiode is zero under the conditions of specific wavelength and a fixed bias voltage. If there is wavelength variation in incident light, the output current is not zero because the two photocurrents generated by these two respective diodes cannot be offset each other. Thus, the wavelength shift can be detected by implementing the output current. However, U.S. Pat. No. 6,678,293 needs specific fabricating parameters, which in turn significantly increases manufacturing cost. Thus, this approach cannot be applied to the LED backlight module. Another prior art is U.S. Pat. No. 7,133,136 that discloses a method for stabilizing wavelength and intensity of laser radiation. This method is achieved by implementing two photodiodes; one is responsible for measuring light intensity and the other is responsible for measuring wavelength. U.S. Pat. No. 7,133,136 has a drawback in that since directivity of LED radiation is not so high as the laser, wavelength variation of LED radiation cannot be sensed by implementing operations at different incident angles of photodiode radiation. All aforementioned prior arts intend to detect the wavelength shift of the laser radiation. Even these prior art are applied to the LED backlight module, they only are capable of identifying color. However, in the LED backlight module, the wavelength variation of the LED radiation is only 1-2 nm, which cannot cause color shift in chromaticity coordinate so that these prior arts cannot be applied to detect this color shift. Moreover, these prior arts cannot be applied to detect every wavelength variation of individual LED in the LED backlight module, and then compensate the wavelength variation for each LED. Accordingly, there exists a need for stabilizing wavelength (or referred to as “stabilizing chromaticity coordinate”) of LED radiation for each LED in backlight module, by using different compensation coefficients for different wavelengths.